Communication system



March 2, 1943.

F. H. KROGER COMMUNICATION SYSTEM Fild Aug. 17, 1940 INVENTOR FRED H.KROGER BY ATTORNEY Fred H. Kroger, Les Radio Corporation of DelawareApplication August 17, 1940, Serial No. 353,004

6 Claims.

This invention relates to a wireless communication system for use oververy short distances. The system of the invention finds particularusefulness in establishing communication between points spaced about onethousand feet apart and less, although it should be understood that itsrange of usefulness is not. limited by this particular distance. 7

Some of the objects of the present invention are:

(1) To provide a wireless communication sys-- tem for use over shortdistances wherein for all points within the range of satisfactorycommunication the rate of decrease .with respect to distance of thesignal power approximates the inverse sixth power of the distance;

(2) To provide a wireless communication system employing audio carrierwaves (for example 9 kc.) and functioning by virtue of the inductionfleld, and without depending on radiation.

FFICE Angela, Calif assignmto of America, a corporation (ca. its-e2) (3)To provide a wireless communication sys- I ter whose magnetic fieldfalls of! as the inverse cube of the distance and whose usefulness-isnot'interfered with by objects in the transmission path.

(4) To provide equipment of the foregoing type which is compact, oflight weight, portable, inexpensive to manufacture, and consumes only asmall amount of power, and does not cause interference to othercommunication systems not using wires.

The invention in its preferred form comprises a battery-operated andportable transceiver which requires only a simple switching operation tochange the system from a transmitting con-' dition to a receivingcondition, and vice versa. For this purpose, the equipment need use onlystandard small vacuum tubes of the batteryoperated type. The term"transceiver is here used to designate a single piece of apparatus whichcan be signal waves.

An essential aspect of the invention resides in the fact that it employsfrequencies which are so low that throughout the distances over whichsatisfactory communication is provided, substantially all of the voltagereceived oy"'a"receiver in accordance with the invention is caused bythe induction held of the transmitter, and the radiated field is sosmall as to be entirely negligible and ir.capable of producinginteri'erence beyond the satisfactory range of communication. lowfrequencies I By means of such relatively am able to make the inductionused both to transmit and receive field greatly exceed tances whichwhile important uses.

The invention finds use in small are sufilcient for many establishingcom- I munication between automobiles traveling in the diiferenttransmitted frequencies to inform the driver of traffic conditions. Theinvention may also be employed at fires so that the firemen can freelyadvise each other and the fire chief of conditions and receiveinstructions. Other uses are in fabricating shops and in laboratoriesand the like, where the center of activity moves about and the layout iscontinually varying, and also in the erection of major size structuresfor the purpose of correlating activities within the range of some 400it. The invention may also be used for military purposes and forcommunication between two houses separated a relatively short distanceapart.

Other objects and aspects of the invention p r in the followingdescription taken in conjunction with a drawing whose single figureshows, by way of example only, one embodiment of the present invention.

In the drawing there is shown a combined transmitting and receivingapparatus (sometimes called a transceiver) which requires only a simpleswitching operation to change the system from a transmitting conditionto a receiving condition, and vice versa. The transceiver of theinvention includes a suitable induction pickup device I, here shown byway of illustration as a large coil, 9. carrier frequency vacuum tubeamplifier 2, a vacuum tube oscillator-detector 3, a signal amplifier 5,a loudspeaker or head-phone set, '5, and a microphone or telephonetransmitter 6. The switches l, 8, 9, l0,- H, i2 and 23 are unicontrclledand when in the full line posltion condition connect the apparatus forreceiving purposes, while in the dotted tion condition connect theapparatus for transmitting purposes. Condensers 24,- 24 are blockingcondensers, and coils 25, 25 are choke coils. The 1 ii i, in combinationwith variable condenser Is, provides a circuit tuned to the carrierfrequency. The plate circuit of the tube 3 includes a parallel tunedcircuit it which is tuned line posithe radiation field up to dis- 1 2 tothe carrier of the waves to be transmitted or received. Connected to oneterminal of the tuned circuit I4 is a suitable network comprising a highresistance l5 and a by-pass condenser 23 in parallel relation. Thisresistance provides a highimpedance to audio frequencies, and if desiredmay be replaced by a suitable choke coil arrangement. The condenser 26provides a bypass (that is, path of very low impedance) for currents ofthe carrier frequency.

The oscillator-detector 3 functions as a detector for the carrierfrequency when used for receiving purposes and as an oscillator whenused for transmitting purposes. In the receiving position and with theswitches Ill, II and I2 in the full line positions indicated in thedrawing, the incoming carrier waves are impressed on the grid of thevacuum tube 3 over lead 16. The output circuit l4 tuned to the carrierfrequency will be short-circuited by switch II and hence only thedemodulated audio frequency components of the received message waveswill flow over lead II to the grid'of the signal vacuum tube amplifier4. The signals amplified by tube 4 will pass over lead l8 to theloudspeaker or head-phones 5. In the transmitting condition, the networkof resistance l5 and by-pass condenser 26 is short-circuited by switchll now in the dotted position, and the carrier frequency tuned circuitl4 functions with the inductance coil l9 coupled thereto to provide afeed-back circuit for the vacuum tube 3 thus producing oscillations ofthe carrier frequency. Grid modulation of the oscillator is effected bymeans of the waves impressed on the grid of tube 3 over a path includingmicrophone 5, switch 1, signal amplifier 4, lead l3, swich 23, audiofrequency transformer 20, coil l9, switch l and lead 2 l.

A brief description of the operation of the system of the inventionillustrated in the drawing will now be given. Assuming that the carrierfrequency to be employed for transmission is 8000 cycles (8 kc.) and itis desired to transmit message waves, the switches 1,. 8, 9, l0, ll, 12and 23 will be in the dotted line position as shown. The voice currentsimpressed on the microphone or telephone transmitter will be transmittedthrough the audio frequency transformer 22 and position to the grid ofthe vacuum tube signal amplifier 4 where they will be amplified and theamplified voice currents passed over lead l8 and switch 23 to thetransformer 20 from which they will flow through the inductance coil I!to the grid of the vacuum tube 3, thus modulating the oscillationsproduced by the oscillator 3. Since the switch H is in the transmittingposition the network of resistance be shorted and thus have no effectwhatever, while the parallel tuned circuit I4 tuned to the carrierfrequency of 8 kc, will have full eflect. The modulated carrierfrequency oscillations from the oscillator 3 will flow over lead l1 andthrough blocking condenser 24 and switch 8 to the grid of the carrieramplifier 2 where they will be amplified and the output of this carrieramplifier applied to the carrier tuned circult composed of the variablecondenser l3 and the coil I. The coil I may be of the loop type asshown, or a solenoid 01f a capacity antenna, and will produce aninduction field which is solely relied upon for purposes ofcommunication. This induction field, in the present case a magneticfield, falls off as the inverse cube of the will flow over switch I inclosed I I5 and condenser 26 will distance from the apparatus of theinvention and consequently is effective for only relatively shortdistances. Putting it another way, it may be said that the signal powerproduced at the transducer decreases with distance at a rateapproximating the inverse sixth power of the distance. Within therelatively short distance within which communication is to be effectedwith a receiver of the type also shown in the drawing, let us say withina thousand feet more or less, the induction field greatly exceeds theradiation field. For distances, however, which are beyond the distanceover which satisfactory communication is intended, the radiated field isso small as to be entirely negligible and incapable of producinginterference with other systems of communication, such as radiofrequency systems. Because of the characteristics of the inductionfield, the usual objects in the environment, such as buildings in thetransmission path between.

the transmitter and receiver, will not cause shadows; that is, thesignals from the transmitter of the invention will fiow around theseobjects and in this way the usefulness of the communication system isnot interfered with.

For receiving purposes, the switches I, 8, 9, l3, ll, l2 and 23 will bein the full line position. The induction field produced by a transmitterof the type shown in the drawing will set up in the receiver tunedcircuit I, I3 a carrier frequency which will be applied to the grid ofthe carrier amplifier 2. The output or amplified carrier frequency fromtube 2 will pass through blocking condenser 24 and over lead l6 andswitch 12 to the grid of the vacuum tube 3 now functioning as adetector. Since the carrier frequency tuned circuit l4 associated with avacuum tube 3 will now be short-circuited, the network of resistance l5and condenser 26 will be fully effective and the demodulated signalfrequency components in the output of the detector will fiow over leadl1 and through blocking condenser 24 to the grid of the signal amplifier4 from which the amplified components will be applied to the loudspeaker5 over a path comprising lead l3 and switch 23.

Although the illustrated embodiment of the invention has been describedfor use with a carrier frequency of 8 kc., it should be distinctlyunderstood that this particular carrier frequency is given by way ofexample only, and thatthe invention is not limited thereto since otherrelatively low frequency carrier frequencies can also be employed, solong as the carrier frequency is low enough to provide an inductivefield which exceeds the radiation field over the distance ofcommunication.

In one experimental embodiment tried out in practice, a carrierfrequency of 11.5 kc. was employed and communication effected at adistance of 600 ft. In this experimental model a coil one .meter squarewith 40 turns of wire was employed for the coil. No carrier frequencyamplifier was used and energy from the loop was fed directly into theoscillator-detector tube 3. It will be obvious that where a loop isemployed it may be used either in the vertical or horizontal plane.

What I claim is: A

1. A transceiver for effecting wireless communication between stationsspaced apart a very short distance, comprising an inductor, an electrondischarge device circuit for generating and impressing upon saidinductor a sinusoidal lowfrequency carrier currentysaid low frequencybeing ofsuch order that at distances from the eluding means for tancewireless prising an impedance device having an induction 'tinuous waveciated withits transceiver where the induction field is smaller than theradiated held the intensity oi said radiated field is too small to causeinterference with other signalling systems, a switch for convetting saidelectron discharge device circuit from a generator of low frequencycarrier current to a detectorof low frequency carrier current, aloudspeaker, a microphone, an additional switch for operativelyassociating either said loudspeaker or said microphone with said device,whereby said loudspeaker is connected to the output of said device whensaid circuit functions as a detector and said microphone is connected tomid device when said circuit functions as'ar oscillator.

2. A system in accordance with claim 1, in-- simultaneously operatingboth of said switching means. 7 1 e i 3. A transceiver for use in everyshort distance wireless system or communication comcoil of which isassociated with its input electrode, a microphone coupled to saidgenerator for modulating the carrier frequency current,

quency current, a

prising an inductor, an electron discharge device circuit for generatingand impressing upon said inductor a sinusoidal audio frequency carriercurrent, a switc for, converting said electron discharge device circuitfrom a generator of audio frequency carrier current'to a detector ofaudio frequency carrier current, a loudspeaker, a microphone, anadditional switch for operatively associating either said loudspeaker orsaid microphone with said device, and means,

linked to 50th said switches for controlling the same simultaneouslywhereby said loudspeaker said device when said circuit functions as adetector and said is connected to the output 0! microphone is connectedto the input of said device when said circuit functions asan se nlatera. transceiver system 0! communication, comtube oscillation generatorim- Drwing on said device an audio frequency concarrier current, saidgenerator in cluding steed-back circuit having a pair of inductivelycoupled coils one coil oi which is asaooutput electrode and the otherfor use in a very short dis-.

an electro-acoustic transducer, and switching means fordisconnectingsaid vacuum tube from said microphone and operativelyassociating said tube with said transducer to serve as a detector ofaudio frequency carrier current received on said device.

5. A transceiver for use in a very short distance wireless system 01'.communication, comprising an impedance device having an induction field,a vacuum tube oscillation generator impressing on said device an audiofrequency continuous wave carrier current, said generator including afeed-back circuit, said generator for modulating the carrier i'resignalamplifier having an input electrode coupled to said means and an outputelectrode coupled to said generator, an

audio frequency translating device. switches associated with the inputelectrode oi said ampliher for disconnecting said input electrode fromsaid modulating means and {or operatively associating it with the outputof said vacuum tube generator, and additional switches for disconnectingthe output electrode of said amplifier operatively assofrom saidgenerator and for ciating it withsaid translating device-and also forconverting said generator 6.'A transceiver for use tance wireless systemof communication, comprising an impedance device having an inductionheld, a vacuum tube oscillation generator impressing on said device anaudio frequency continuous wave-carrier current, said generatorineluding an external feed-back circuit, a microphone coupied to saidgenerator for modulating the carrier frequency current, anelectro-acoustic transducer, and ing said vacuum operatively ducer toserve as a detector of switching means for disconnecttube from saidmicrophone and associating said tube with said transaudio frequency onsaid device.

carrier current received means coupled to to a detector 0: ,audiofrequency carrier current.

in a very short dic-

